the sea itself has not been satisfactorily solved. This is essen- 

 tially a sampling problem and is inherent in all observational data. 

 These problems are not restricted to survey observations, but are 

 of greater concern in surveys since the comparability of data taken 

 by standardized procedures over the oceans of the earth is an im- 

 portant goal. 



Oceanographic instruments can be grouped into two general 

 categories: Sensing instruments and capturing instruments. 

 SENSING INSTRUMENTS are used to obtain measurements by 

 light or sound or some other attribute of the thing measured; a 

 CAPTURING INSTRUMENT is used to capture the thing to be mea- 

 sured, and the measuring generally follows capture. 



Examples of visual sensing systems are submerged autonnatic 

 photographic or television cameras. Variations in the light absorp- 

 tion of the water itself has biological qonnotation, especially when 

 taken well off shore on the high seas. Also, such a simple thing 

 as an experienced man with a good pair of binoculars on his ves- 

 sel's bridge sweeping the sea for evidence of surface schools of 

 fish, can provide quantitative data. We used two divers to count 

 fish observed along a line laid on the sea floor in Hawaii to esti- 

 mate quantatively the abundance of inshore fishes by species and 

 size. The object here was to compute the weight of the fish stocks 

 per unit area of sea floor. This is, perhaps, a specialized tech- 

 nique but biologists have complicated problems, and tend to 

 develop special ways to get the infornnation they want. 



In addition to visual sensing systems, there are sonic systems. 

 Passive listening devices, such as the hydrophone, may be useful; 

 active devices employ echo ranging equipment. 



The other group is the capturing instruments. The simplest 

 is the Nansen bottle or others of this type which collect a sample 

 of water at a preselected depth. The Nansen bottle takes water 

 for physical measurements such as temperature, or chemical 

 measurements such as salinity or oxygen. Included with the water 

 samples are animals or plants that happen to be in it. 



Next is the plankton net which has been generally used for a 

 long time by biologists as a standard instrument. It is generally 

 used with a flow meter in the mouth. In some cases it is modified 

 so that it can be opened and closed at various depths. A clever 

 development of the plankton net is the continuous plankton recorder 



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